This invention relates to the art of synthesizing aminophenols.
Metal catalyzed hydrogenation of halo-dinitro-dihydroxy benzenes is known to be useful for synthesizing diaminobenzenediols, such as 4,6-diaminoresorcinol, which are useful as monomers for polybenzoxazole polymers. See Lysenko, High Purity Process for the Preparation of 4,6-Diamino-1,3-benzenediol, U.S. Pat. No. 4,766,244 (Aug. 23, 1988) which is incorporated herein by reference. During hydrogenation, the halogen is removed from the aromatic ring as hydrogen halide.
Halogen is difficult to remove by hydrogenation from the aromatic ring of aminophenol compounds when the aminophenol moiety is protonated. Hydrogenation of halo-nitro-phenolic compounds is ordinarily carried out under basic conditions, so that the amine group produced by hydrogenation of the nitro group is not protonated. However, the free base of the aminophenolic product is unstable with respect to oxidation in aqueous media, so the hydrogenation is ordinarily carried out in organic solvents. See U.S. Pat. No. 4,766,244 at column 3, lines 52-59. Organic solvents are very costly to use in commercial scale production. Furthermore, organic solvents can present an undesirable fire risk, particularly in the presence of flammable hydrogenation catalysts and potentially explosive nitrated phenols. What is needed is a high yield process for the synthesis of aminophenolic compounds which can be practiced in an aqueous medium containing either reduced levels of organic solvent or no organic solvent at all.